The present invention relates to soldered connections for microelectronic devices such as semiconductor chips and the associated circuit panels, and to methods of making and using such connections.
Microelectronic circuits require numerous connections between elements. For example, a semiconductor chip may be connected to a small circuit panel or substrate, whereas the substrate may in turn be connected to a larger circuit panel. The chip to substrate or "first level" interconnection requires a large number of individual electrical input and output ("IO") as well as power and ground connections. As chips have become progressively more complex, the number of I/O connections per chip has grown so that hundreds of connections or more may be needed for a single chip. To provide a compact assembly, all of these connections must be made within a relatively small area, desirably an area about the area of the chip itself. Thus, the connections must be densely packed, preferably in an array of contacts on a regular grid, commonly referred to as a "Bump Grid Array" or "BGA". The preferred center-to-center distance between contacts or "contact pitch" for chip mountings is on the order of 1.5 mm or less, and in some cases as small as 0.5 mm. These contact pitches are expected to decrease further. Likewise, chip mounting substrates and other circuit panels used in microelectronics have become progressively more miniaturized, with progressively greater numbers of electrical conductors per unit area. Connectors for these miniaturized panel structures desirably also have very small contact pitch. Connections of chip mounting substrates to other elements are referred to as "second-level" interconnections.
Microelectronic connections must meet numerous, often conflicting requirements. As mentioned above, the size of the device poses a major concern. Further, such connections often are subject to thermal cycling strains as temperatures within the assembly change. The electrical power dissipated within a chip or other microelectronic element tends to heat the elements so that the temperatures of the mating elements rise and fall each time the device is turned on and off. As the temperatures change, the various connected elements expand and contract by different amounts, tending to move the contacts on one element relative to the mating contacts on the other element. Changes in the temperature of the surrounding environment can cause similar effects which produce mechanical stress in the connected components.
The connections must also accommodate manufacturing tolerances in the contacts themselves and in the connected elements. Such tolerances may cause varying degrees of misalignment. Additionally, contamination on the surfaces of the mating contact parts can interfere with the connection. Therefore, the contact system should be arranged to counteract the effects of such contaminants. For example, in making soldered connections, oxides and other contaminants must be removed by fluxes. These fluxes in turn can contaminate the finished product. Although these fluxes can be removed by additional cleaning steps, or can be formulated to minimize ill-effects on the finished product, it would be desirable to provide soldered connections which minimize or eliminate the need for Such fluxes. All of these requirements, taken together, present a formidable engineering challenge.
Various approaches have been adopted towards meeting these challenges.
Certain preferred embodiments disclosed in our aforementioned U.S. patent application No. 08/254,991 provide connectors for mounting a microelectronic element such as a semi-conductor chip or other element. Connectors according to these embodiments include a planar dielectric body having first and second surfaces and also having a plurality of holes open to the first surface. The holes are disposed in an array corresponding to an array of bump leads on the device to be mounted. The connector further includes an array of resilient contacts secured to the first surface of the dielectric body in registration with the holes so that each such contact extends over one hole. Each contact is adapted to resiliently engage a bump lead inserted into the associated hole. A chip or other microelectronic component with the bump leads thereon can be connected to the contacts by superposing the microelectronic element on the dielectric body of the connector so that the microelectronic element overlies the first surface and so that the bump leads on the element protrude into the holes and are engaged by the resilient contacts. Preferred connector components according to this aspect of the invention will establish electrical connection with the bump leads by mechanical inter-engagement of the bump leads and contacts.
Each contact may include a structure such as a ring of a sheet-like metallic contact material overlying the first surface of the dielectric body and fully or partially encircling the opening of the associated hole, and each contact may also include one or more projections or tabs formed integrally with the ring and extending inwardly therefrom over the hole. Preferably, a plurality of such projections are provided at circumferentially-spaced locations around the hole. These projections are arranged so that when a bump lead enters the hole, it tends to force the projections downwardly and outwardly, away from one another. The projections tend to center the bump in the hole. The chip or other microelectronic component can be reliably connected simply by pressing the chip against-the connector in proper alignment with the holes. This reliable interconnection can be used either as a temporary interconnection for testing purposes or as a permanent connection.
As set forth in the No. '991 application, the motion of the bump leads entering the holes as the microelectronic element is engaged with the connector causes the bump leads to wipe across the contacts so as to clean debris, oxides and other contaminants from the surfaces of the contact and bump lead. The bump leads may include a bonding material such as a solder, to form a permanent metallurgical connection with the contacts. Thus, the microelectronic component can be engaged with the connector and tested using the mechanically-made electrical interconnections. If the results are satisfactory, the permanent metallurgical bond can be formed by heating to melt the solder.
Certain aspects of our U.S. patent application No. 08/306,205 provide contacts for a microelectronic device, which contacts can be used in the connectors of the No. '991 application and in other structures. According to these aspects of the No. '205 application, each contact includes a base portion defining a base surface, and one or more asperities preferably integral with the base portion and protruding upwardly from the base surface. Each such asperity desirably defines a tip remote from the base surface and a substantially sharp feature at the tip. The base portion of each contact may include one or more metallic layers such as copper or copper-bearing alloys, and may also include a polymeric structural layer in addition to a conductive, desirably metallic, layer.
The base portion of each contact may include an anchor region and at least one tab or projection formed integrally with the anchor region. The asperity or asperities may be disposed on each tab at a distal end, remote from the anchor region. In use, the anchor region of such a contact is fixed to a connector body or other support, whereas the tab is free to bend. When a bump lead is engaged with the tab, the tab bends and the mating bump lead and tab move relative to one another to provide a wiping motion. The resilience of the tab causes the sharp feature of the asperity to bear on the mating element and scrape the mating element. The scraping action promotes reliable contact before bonding, as well as reliable bonding. The anchor region of each contact may be part of a substantially ring-like common anchor region. A contact unit may include such common anchor region and a plurality of tabs extending inwardly from the ring-like anchor region towards a common center.